Origin

When the souls of the dead in Greek mythology drank the water of Lethe, a river in Hades, the underworld, they forgot their life on earth, and so in Greek the word Lēthē meant ‘forgetfulness’. Many ancient Romans were familiar with this, and along the line they altered their Latin word letum ‘death’ to lethum, to be closer to the Greek. The altered Latin form is the source of English lethal ‘deadly’. Lethargy (Late Middle English) comes from a related Greek word.